1. Field of the InventionThis invention pertains in general to ion conductors and to processes for the synthesis of ion conducting solid electrolytes. In particular, the invention relates to the use of nanoscale powders for the preparation of nanostructured oxygen ionconducting electrolytes.2. Description of the Prior ArtSolid electrolytes are materials through which ion species can migrate with low energy barriers. Table 1 outlines some examples of ion-conducting structures, representative materials, and the ions conducted. These materials are of criticalcommercial importance to electrochemical devices, components and processes. Illustrative applications include sensors, batteries, fuel cells, ion pumps, membrane reactors, catalysis, and metallurgy. REPRESENTATIVE MATERIALS ION CONDUCTED Stabilized ZrO.sub.2 System, Stabilized Bi.sub.2 O.sub.3 O.sup.2- System, Ceria, Perovskites Beta-Alumina, NASICON Systems Na.sup.+ AgI, RbAg.sub.4 I.sub.5 Ag.sup.+ Rb.sub.4 Cu.sub.16 I.sub.7 Cl.sub.13Cu.sup.- Li.sub.3 N, Li.sub.2 S--SiS.sub.2 --Li.sub.3 PO.sub.4 System, Organic Li.sup.+ Polymer Systems, LISICON SystemsAs a specific example, stabilized zirconia is a known conductor of oxygen ions. Accordingly, its properties are utilized in various fields of technology, such as in oxygen sensors for fuel-air ratio optimization of automobiles and furnaces, inoxygen pumps for solid state oxygen separation, in solid-oxide fuel cells for noiseless and clean power generation from chemical energy, and in catalytic membrane reactors.The oxygen-ion conduction properties of stabilized zirconia used in a typical oxygen sensor are well understood based on electrochemical-cell theory. When placed between two compartments containing a reference gas and an analyte oxygen gas atdifferent partial pressures, stabilized zirconia functions both as a partition between the two compartments and as an electrochemical-cell electrolyte. Under ideal conditions, the open-circuit emf (E.sub.0

United States Patent: 6387560
&nbsp;
( 1 of 1 )
United States Patent
6,387,560
Yadav
, &nbsp; et al.
May 14, 2002
Nanostructured solid electrolytes and devices
Abstract
Ion conducting solid electrolytes are constructed from nanoscale precursor
material. Nanocrystalline powders are pressed into disc structures and
sintered to the appropriate degree of densification. Metallic material is
mixed with 0 to 65 vol % nanostructured electrolyte powders to form a
cermet mix and then coated on each side of the disc and fitted with
electrical leads. The electrical conductivity of a Ag/YSZ/Ag cell so
assembled exhibited about an order of magnitude enhancement in oxygen ion
conductivity. As an oxygen-sensing element in a standard O.sub.2
/Ag/YSZ/Ag/N.sub.2 set up, the nanocrystalline YSZ element exhibited
commercially significant oxygen ion conductivity at low temperatures. The
invention can be utilized to prepare nanostructured ion conducting solid
electrolytes for a wide range of applications, including sensors, oxygen
pumps, fuel cells, batteries, electrosynthesis reactors and catalytic
membranes.
Inventors:
Yadav; Tapesh (Tucson, AZ), Hu; Hongxing (Tucson, AZ)
Assignee:
Nano Products Corporation
(Longmont,
CO)
Appl. No.:
09/251,313
Filed:
February 17, 1999
Related U.S. Patent Documents
Application NumberFiling DatePatent NumberIssue Date
739257Oct., 19965905000
730661Oct., 19965952040
706819Sep., 19965851507Dec., 1998
707341Sep., 19965788738Aug., 1998
Current U.S. Class:
429/484 ; 204/424; 204/426; 429/304; 429/488; 429/489; 429/505; 429/533
Current International Class:
B01J 19/24&nbsp(20060101); B01J 12/00&nbsp(20060101); B01J 12/02&nbsp(20060101); B22F 9/02&nbsp(20060101); B22F 9/12&nbsp(20060101); C01B 31/00&nbsp(20060101); C04B 35/622&nbsp(20060101); C01B 35/04&nbsp(20060101); C04B 41/45&nbsp(20060101); C04B 41/89&nbsp(20060101); C01F 11/06&nbsp(20060101); C01B 31/36&nbsp(20060101); C01F 17/00&nbsp(20060101); C01B 13/14&nbsp(20060101); C04B 2/00&nbsp(20060101); C04B 41/52&nbsp(20060101); C01B 35/00&nbsp(20060101); C01G 23/00&nbsp(20060101); C01F 5/00&nbsp(20060101); C01F 5/06&nbsp(20060101); C01F 11/00&nbsp(20060101); C01B 19/00&nbsp(20060101); C01B 21/00&nbsp(20060101); C04B 2/10&nbsp(20060101); C01B 21/06&nbsp(20060101); C01G 53/00&nbsp(20060101); C01G 41/00&nbsp(20060101); C01G 41/02&nbsp(20060101); H01G 4/33&nbsp(20060101); H01C 7/112&nbsp(20060101); H01G 4/12&nbsp(20060101); H01C 7/105&nbsp(20060101); H01M 8/12&nbsp(20060101); H01M 004/88&nbsp(); H01M 010/39&nbsp(); G01N 027/407&nbsp()
Field of Search:
429/33,45,304,320,321,322 204/421,422,424,426
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Primary Examiner: Kalafut; Stephen
Attorney, Agent or Firm: Langley; Stuart T.
Hogan & Hartson LLP
Parent Case Text
RELATED APPLICATIONS
This application is a continuation of U.S. Ser. No. 08/739,257, filed Oct.
30, 1996, now U.S. Pat. No. 5,905,000, which is a continuation-in-part of
U.S. Ser. No. 08/730,661, entitled "Passive Electronic Components from
Nano-Precision Engineered Materials," filed on Oct. 11, 1996, now U.S. Pat
No. 5,952,040, which is a continuation-in-part of U.S. Ser. No.
08/706,819, filed Sep. 3, 1996 entitled "Integrated Thermal Process and
Apparatus for the Continuous Synthesis of Nanoscale Powders," now issued
as U.S. Pat. No. 5,851,507 on Dec. 22, 1998, and U.S. Ser. No. 08/707,341,
entitled "Boundary Layer Joule-Thompson Nozzle for Thermal Quenching of
High Temperature Vapors," filed concurrently on Sep. 3, 1996, now issued
as U.S. Pat. No. 5,788,738 on Aug. 4, 1998. These applications and patents
are all commonly owned with the present application, and are all
incorporated by reference herein.
Claims
We claim:
1. An electrochemical comprising
a porous nanocomposite solid electrode having a first and a second surface, comprising a conductive material and nanoscale particles of an electrolyte material and having a pore fraction in the range of 15-50 percent,
wherein an electrical potential is developed between the first and second surfaces when the first surface is exposed to a reference species and the second surface is exposed to an analyte species.
2. The device of claim 1, wherein the device is a gas sensor and the analyte species is selected from the group consisting of monatomic oxygen, diatomic oxygen, and ozone.
3. An electrochemical device, comprising a nanostructured electrolytic structure having an first surface and a second surface, prepared by
aggregating nanoscale particles of a ceramic electrolyte material to form the electrolytic structure; and
sintering the electrolytic structure at a temperature sufficient to cause its densification, said temperature being less than 1000.degree. C.,
wherein an electrical potential is developed between the first and second surfaces when the first surface is exposed to a reference species and the second surface is exposed to an analyte species.
4. The device of claim 3, wherein the device is a gas sensor and the analyte species is selected from the group consisting of monatomic oxygen, diatomic oxygen, and ozone.
5. The device of claim 1 or 3, wherein the device is a gas sensor and the analyte species is a gas.
6. The device of claim 1 or 3, wherein the device is a chemical sensor and the analyte species is dissolved oxygen.
7. The device of claim 1 or 3, wherein the device is a chemical pump.
8. The device of claim 1, wherein the conductive material is silver.
9. The device of claim 1 or 3, wherein the electrolyte material is selected from the group consisting of yttria stabilized zirconia and yttria stabilized bismuth oxide.
10. The device of claim 1, wherein the electrode has a resistance which is less than or equal to about a fourth of the resistance of an electrode composed purely of the conductive material.
11. A solid electrolyte based device, comprising
a nanostructured electrolytic structure, prepared by
aggregating nanoscale particles of a ceramic electrolyte material to form the electrolytic structure; and
sintering the electrolytic structure at a temperature sufficient to cause its densification, said temperature being less than 1000.degree. C.,
wherein the device is selected from the group consisting of an energy storage device, an energy generation device, a device for thermodynamic property measurements, a device for electrocatalysis, a device for electrosynthesis, a solid state
chemical pump, and a chemical sensor.
12. The device of claim 11, wherein the energy generation device is a battery.
13. The device of claim 11, wherein the energy generation device is a fuel cell.
14. The device of claim 11, further comprising a porous nanocomposite solid electrode comprising a conductive material and nanoscale particles of an electrolyte material and having a pore fraction in the range of 15-50 percent.
15. The device of claim 11, wherein the ceramic electrolyte material is stabilized bismuth oxide. Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains in general to ion conductors and to processes for the synthesis of ion conducting solid electrolytes. In particular, the invention relates to the use of nanoscale powders for the preparation of nanostructured oxygen ion
conducting electrolytes.
2. Description of the Prior Art
Solid electrolytes are materials through which ion species can migrate with low energy barriers. Table 1 outlines some examples of ion-conducting structures, representative materials, and the ions conducted. These materials are of critical
commercial importance to electrochemical devices, components and processes. Illustrative applications include sensors, batteries, fuel cells, ion pumps, membrane reactors, catalysis, and metallurgy.
REPRESENTATIVE MATERIALS ION CONDUCTED Stabilized ZrO.sub.2 System, Stabilized Bi.sub.2 O.sub.3 O.sup.2- System, Ceria, Perovskites Beta-Alumina, NASICON Systems Na.sup.+ AgI, RbAg.sub.4 I.sub.5 Ag.sup.+ Rb.sub.4 Cu.sub.16 I.sub.7 Cl.sub.13
Cu.sup.- Li.sub.3 N, Li.sub.2 S--SiS.sub.2 --Li.sub.3 PO.sub.4 System, Organic Li.sup.+ Polymer Systems, LISICON Systems
As a specific example, stabilized zirconia is a known conductor of oxygen ions. Accordingly, its properties are utilized in various fields of technology, such as in oxygen sensors for fuel-air ratio optimization of automobiles and furnaces, in
oxygen pumps for solid state oxygen separation, in solid-oxide fuel cells for noiseless and clean power generation from chemical energy, and in catalytic membrane reactors.
The oxygen-ion conduction properties of stabilized zirconia used in a typical oxygen sensor are well understood based on electrochemical-cell theory. When placed between two compartments containing a reference gas and an analyte oxygen gas at
different partial pressures, stabilized zirconia functions both as a partition between the two compartments and as an electrochemical-cell electrolyte. Under ideal conditions, the open-circuit emf (E.sub.0) of the cell is given by the known Nernst
equation: ##EQU1##
where T is the absolute temperature of the cell; PO.sub.2.sup.Ref, and PO.sub.2 are the partial pressures of oxygen in the reference and analyte compartments, respectively; R is the universal gas constant; and F is Faraday's number.
According to this equation, any difference in partial pressure of the oxygen across the two faces of the oxygen-conducting electrolyte generates an electromotive force that depends on the temperature and partial-pressure ratio of the oxygen in
the two compartments of the system. In order to generate Nernstian response in sufficiently short times, the temperature of stabilized ZrO.sub.2 needs to be high (above 700.degree. C.), which results in relatively high power requirements and in
increased equipment mass and size, need for insulation, and attendant sealing problems. These considerations often produce unsatisfactory performance or affect the commercial viability of products based on stabilized ZrO.sub.2 technology.
The inherent reasons for the high-temperature requirement and the corresponding performance problems of present-day oxygen ion conducting electrolyte based devices can be traced to the reaction mechanism of the cell and the microstructure of the
sites where the reaction occurs. Referring to FIG. 1, a schematic drawing of a ZrO.sub.2 sensor cell 10 is illustrated, where the stabilized zirconia is modeled as a solid electrolyte membrane 12 between a first compartment 14, containing a reference
oxygen atmosphere at a predetermined partial pressure PO.sub.2.sup.Ref, and another compartment 16 containing an analyte gas with oxygen at a different partial pressure PO.sub.2. The two sides of the stabilized zirconia non-porous solid electrolyte 12
are coupled through an external circuit connecting an anode 18 and a cathode 20 made of porous metal, such as silver. The anode 18 is the cell electrode at which chemical oxidation occurs and the electrons released by the oxidation reaction flow from it
through the external circuit to the cathode. The cathode 20 is the cell electrode at which chemical reduction occurs. The cell electrolyte 12 completes the electrical circuit of the system by allowing a flow of negative ions O.sup.2- between the two
electrodes. A voltmeter 22 is provided to measure the emf created by the redox reactions occurring at the interfaces of the electrolyte with the two oxygen atmospheres.
Thus, the key redox reaction of the cell occurs at the points where the metal electrode, the electrolyte and the gas meet (illustrated in the inset of FIG. 1 as the "triple point" 24 ). At each such site on the surface of the electrolyte 12, the
redox reaction is as follows:
Since the reaction and the electrochemical performance of the sensor depend on the redox kinetics, the cell's performance is a strong function of the concentration of triple points. In other words, an electrode/electrolyte/electrode cell with as
many triple points as possible is highly desirable [see Madou, Marc and M. Morrison, Chemical Sensing with Solid State Devices, Academic Press, Boston (1989)]. In the case of an oxygen cell with a ZrO.sub.2 solid membrane and silver electrodes, this
requirement corresponds to maximizing the triple points on each side of the PO.sub.2.Ag'/ZrO.sub.2 /Ag"PO.sub.2.sup.Ref system.
Another cause of poor performance of oxygen-sensor cells can be explained with the help of complex-impedance analysis. Referring to FIGS. 2a and 2b, a complex impedance diagram for a ZrO.sub.2 sensor is shown, where the impedances of the bulk,
grain boundary and electrode are illustrated in series to reflect their contribution to the ionic conduction at each triple point. It has been shown that the conductive performance of electrolytes at temperatures below 500.degree. C. is controlled by
the grain boundary contribution to the overall impedance. Thus, for significant improvements of the conductivity at low temperatures, it is necessary to significantly minimize the grain-boundary (interface) resistance.
In summary, oxygen ion conducting devices based on stabilized-zirconia electrolyte have two problems that can be traced to material limitations. First, the electrolytes have high impedance; second, the concentration of triple points is
relatively low. These problems are common to solid oxygen-conducting electrolytes in particular and solid electrolytes in general, and any improvement in these material characteristics would constitute a significant technological step forward. The
present invention provides a novel approach that greatly improves these aspects of ion conducting solid electrolytes.
SUMMARY OF THE INVENTION
One of the objectives of this invention is to enhance the ion-conductivity of solid electrolytes by preparing nanostructured solid electrolytes.
Another objective is to reduce the electrolyte thickness with the use of nanostructured precursors of solid electrolytes.
A further objective is to enhance the concentration of triple points in the ion conducting devices by using nanostructured precursors and materials.
Yet another objective of the invention is to utilize the unique properties of size confinement in solid electrolyte and electrode grains when the domain is confined to less than 100 nanometers.
Another objective of this invention is an oxygen-conducting electrolyte material with low-impedance oxygen conducting characteristics.
Another objective of the invention is an oxygen ion conducting device with a very high density of triple points.
Another goal is a process and materials that reduce the cost of manufacture of products that incorporate oxygen-ion conductors.
Yet another goal is a process and materials that reduce the cost of operation of products that incorporate oxygen-ion conductors.
Finally, another goal is a process that can be readily incorporated with conventional methods for manufacturing products containing ion-conducting electrolytes.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the
objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and
description disclose only some of the various ways in which the invention may be practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of a ZrO.sub.2 solid electrochemical cell where the stabilized zirconia is modeled as a solid electrolyte membrane sandwiched between a first compartment containing a reference oxygen atmosphere at a predetermined
partial pressure and another compartment containing an analyte gas with oxygen at a different partial pressure.
FIGS. 2a and 2b are a complex impedance diagram for a ZrO.sub.2 sensor, where the impedances of the bulk, grain boundary and electrode are illustrated to reflect their contribution to the ionic conduction at each triple point shown in FIG. 1.
FIG. 3 is an X-ray diffraction pattern of the nanoscale yttria stabilized zirconia precursor used to form an electrolyte membrane according to the invention.
FIG. 4 is a transmission electron microscope image of the nanoscale yttria stabilized zirconia nanopowder produced in Example 1.
FIG. 5 is a graph of total conductivity versus temperature of 9-YFSZ nanozirconia prepared by the process of Example 1 and micron-based YFSZ material.
FIG. 6 is a voltage-versus-temperature graph of a nanostructured oxygen sensor manufactured with the material prepared in Example 1. The symmetrical response about the abscissa relates to switching the gases from one face of the sensor to the
other.
FIG. 7 is a flow chart of the process for preparing a yttria stabilized bismuth oxide (YSB) nanopowder from nitrates by a solution method.
FIG. 8 is an X-ray diffraction pattern of the nanostructured YSB product of Example 2.
FIG. 9 is a transmission electron microscope image of the nanostructured YSB produced in Example 2.
FIG. 10 is a scanning electron microscope (SEM) image of the composite structure resulting from the densification of the Ag phase inhibited by an ion electrolyte material phase of Example 3.
FIG. 11 illustrates Nyquist plots of impedance spectra of cells with the nanocomposite electrodes of Example 3 in comparison with a cell with pure Ag electrodes.
FIG. 12 shows electrode resistances of a pure Ag electrode and nanocomposite electrodes as a function of temperature, as determined from the impedance spectra measured in air.
FIG. 13 is the voltage response in oxygen of the sensor of Example 3 as a function of temperature in comparison with that of a conventional YSZ sensor.
FIG. 14 shows a comparison of the ionic conductivity of nanostructured YSB electrolyte with YSZ electrolyte.
FIG. 15 is a flow diagram of the steps of deposition over a supporting substrate according to known vapor deposition processes applicable to the manufacture of nanostructured electrolytes according to the invention.
DETAILED DESCRIPTION
OF THE INVENTION
This invention is based on the recognition that the ion conductivity of polycrystalline solid electrolytes at moderate and near-ambient temperatures is mainly controlled by the conductivity of grain boundary and the concentration of triple
points. The invention further notes that the engineering of grain boundary resistance and triple points in solid electrolyte devices is limited by the electrolyte thickness and electrode characteristics, respectively, which in turn depend on grain size
of the precursors and the material used in the manufacture of electrolytes and electrodes for solid ion conductors in general, and solid oxide oxygen sensors, solid oxide oxygen pumps and solid oxide fuel cells in particular. These limitations
constitute an inherent obstacle to achieve significant technological improvements.
The finest powders currently available for commercial use consist of particles with sizes in the order of several microns. For example, the YSZ powders that are presently used to produce oxygen sensors have an average grain size of about 1 to 3
microns. Since the number of triple points occurring within a given area at the interface with the oxygen atmosphere is necessarily limited by the number of electrode grains distributed within that area, the grain size in the electrode is very important
for maximizing redox-reaction sites. Similarly, since we know that the impedance of the system is reduced by electrolyte thickness, it follows that thinner electrolytes produced from smaller grains would produce lower impedance. Accordingly, the heart
of this invention consists of using nanosize materials in the manufacture of electrolytes for these applications.
The current inability to improve the performance of solid ion conductors is a result of the inability of prior-art processes to economically reduce powder size of precursor materials beyond the micron-size range. Accordingly, the present
invention is based on the work disclosed in commonly-owned U.S. Pat. Nos. 5,851,507 and 5,788,738, which provide a viable vehicle for manufacturing nanoscale powders suitable for the present invention. Material having with physical properties as
produced by the process and apparatus described therein is a necessary ingredient for practicing this invention on a commercial scale.
As defined in the art, submicron powders are materials having average grain size below 1 micrometer. Of critical interest for this invention are nanoscale powders and nanostructured layers of ceramics and electrodes. Nanoscale powders
(nanopowders) are submicron powders with average grain size less than 100 nanometers (preferably with a standard deviation of less than about 25 nm) and with a significant fraction of interfacial atoms. Accordingly, reference to nanoscale powders in
this disclosure is intended to refer to powders with those characteristics.
Submicron layers are layers having thickness less than 1 micrometer. Of particular interest to this invention are nanostructured layers which are defined specifically as layers with thickness, or microstructure, or both, confined to a size less
than property confinement size (positively less than 1 micron, preferably below 100 nm). Accordingly, reference to nanostructured layers in this disclosure is also intended to refer to layers with those characteristics.
As discussed in the copending applications, it is known that within these size ranges a variety of confinement effects occur that dramatically change the properties of the material. The idea of this invention then is to build ion conducting
solid electrolytes from powders whose grain size has been confined to dimensions less than 100 nanometers. The size confinement effects in nanometer scale can confine fundamental processes to band-gap and quantum confined states which in turn can
dramatically change the properties and performance of the resulting solid electrolyte. This insight can be implemented as devices prepared with one dimensional quantum dot and nanocluster composite with the dot size less than 100 nm (preferably less
than 10 nm), as quantum wires with diameter less than 100 nm (preferably less than 10 nm), as quantized and nanoscale films with film thickness less than 100 nm, as nanostructured layers and pellets with microstructure less than 100 nm, and as a
combination of these. In summary, another aspect of the invention concerns the preparation of solid electrolyte and electrodes that are nanostructured.
Nanostructured ion conducting electrolytes prepared from nanostructured materials have grain sizes spatially confined to less than 100 nanometers, a significant fraction (20-60%) of their atoms is interfacial, and exceptional interactions occur
between the constituent domains. Therefore, nanostructured oxygen-conducting electrolytes can be expected to have very high concentrations of interface area which can assist rapid and low-temperature densification of ion conducting electrolytes. The
nanoscale powder can also enable dramatic reduction in layer thicknesses as discussed in co-pending applications. Furthermore, since nanostructure provides higher density of surface area, the density of triple points at the electrolyte-electrode-gas
interface can also be significantly enhanced using nanostructured-electrolyte/electrode interactions. Given low resistance and high triple-point concentration, nanostructured electrolytes and electrodes can be used to achieve higher ion conductivity and
electrochemical activity. This is of particular interest when an ion conducting device has to operate at near ambient temperatures. This general design principle is applicable to all, solid ion conductors based on ion defect structure, two dimensional
layered structure, three dimensional network structure, vitreous structure, .alpha.-AgI type structure, and composites prepared using these structures. Illustrative examples include, without limitation, oxide ion conductors such as stabilized zirconia,
stabilized ceria, stabilized bismuth oxide, perovskites, LISICON, NASICON, and .beta.-alumina.
The following examples illustrate different ways of reducing the present invention to practice.
EXAMPLE 1
A stock solution was prepared from ZrOCl.sub.2.8H.sub.2 O and 9 mol % Y.sub.2 O.sub.3 in water, and diluted with denatured ethanol. The solution was chilled to 0.degree. C. and then slowly added to a continuously stirred basic solution of
ammonium hydroxide that was also maintained at 0.degree. C. Precipitation of white precursor powder was instantaneous. The precipitate solution was suction filtered, and the gelatinous filter cake was washed in denatured ethanol three times. The loose
powder so generated was dried quickly with mild heating at 100.degree. C. to remove water and ethanol, and calcined to 500.degree. C. in air to form nanocrystallites with grain size of about 5.8 nm, standard deviation of 1.1 nm. This precursor
material consisting of 9 mole-percent yttria stabilized zirconia (YSZ) nanoscale powders was examined using an X-ray diffractometer (XRD). A typical XRD pattern for the 9 mole powders so produced is illustrated in FIG. 3, which shows that the ZrO.sub.2
is stabilized cubic phase. In order to determine the average particle size of the powders, the widths of strong, low order peaks of XRD pattern were analyzed using Scherrer's method. The average particle size of the powders according to this analysis
was found to be about 4.5 nanometers. The particle size was also verified by transmission electron microscopy (TEM). FIG. 4 is the resulting TEM image suggesting a particle size of 5.8 nanometers.
The nanoscale 9 mole % yttria stabilized cubic zirconia powders were pressed into 3 mm diameter discs (0.15 gram weight) and sintered to high densities (preferably more than 90% of theoretical density for mechanical strength, over 95% being
preferred). The sample disks were sintered at low temperatures (1,150 to 1,250.degree. C., yielding more than 95% density) and for short duration (6 to 24 hours) to minimize grain growth. We found that YSZ nanopowders readily sintered to full
theoretical densities at about 1,200.degree. C. in 17 hours, which represent significantly milder and less expensive processing conditions than presently necessary. Careful control of the sintering profile and time can further reduce the sintering
temperature and time. The cylindrical discs were examined under XRD and the post-sintered mean grain size by Scherrer analysis was found to be about 83 nm, confirming that the discs were nanostructured. The two ends of the cylindrical discs so produced
were then coated with a cermet paste consisting of a mix of silver and nanoscale yttria stabilized zirconia powder (about a 50--50 wt % mix, corresponding to a 35Ag-65YSZ vol % mix). Then platinum leads were attached to the cermet layer.
The samples were placed in a furnace and their impedance was measured in air as a function of increasing temperature with a computerized impedance analyzer. A standard 40 mV AC bias and frequency sweep range of 5 Hz to 13 MHz were used. As
illustrated in FIG. 5, the results so obtained suggest that nanostructured oxygen-conducting electrolytes, referenced by n, exhibit almost an order of magnitude higher oxygen-ion conductivities at lower temperatures when compared with base-line
electrolytes, referenced by .mu. (i.e., conventional micron-powder based oxygen-conducting electrolytes). It is noted that neither the base-line nor the nanostructured electrolytes represent optimal performance. For additional electrochemical and
electrocatalytic performance evaluation, the Ag/YSZ/Ag cell was tested as a sensor and oxygen pump. For sensor/fuel-cell experiments, oxygen containing gas was passed over one face of the sensor and nitrogen was passed over the other face of the sensor. The emf response as a function of temperature was measured. As shown in FIG. 6, the results indicate that the sensor signal for each gas combination is linear with temperature, confirming a Nernst-type behavior. For electrosynthetic oxygen generation
and pumping applications, CO.sub.2 was passed over one face while nitrogen was passed over the other face. FIG. 6 shows that the oxygen-conducting electrolyte exhibited oxygen pumping properties at low temperatures.
EXAMPLE 2
Bismuth nitrate (Bi(NO.sub.3).sub.3.5H.sub.2 O) and yttrium nitrate (Y(NO.sub.3).6H.sub.2 O), were used as precursors for preparing nanosized yttria stabilized bismuth oxide (YSB) powder via solution co-precipitation. FIG. 7 shows a flow chart
of the co-precipitation processing steps used in this example. After precipitation, the precipitate solution was suction filtered, and the gelatinous filter cake was washed in acetone to minimize agglomeration of ultrafine powder due to hydrogen
bonding. The loose powder so generated was next dried with mild heating to remove water and acetone. Then the powder was calcined in air at 500.degree. C. for 2 hours. XRD showed that calcine schedule resulted in a single cubic YSB phase (see FIG.
8). The volume averaged crystallite size of the powder fired at 500.degree. C. was determined to be 12.5 nm by analyzing the broadening of the (111) diffraction peak and applying Scherrer's formula. The YSB nanopowder was characterized in terms of
morphology and particle size by transmission electron microscopy (TEM). FIG. 9 is a TEM image of the nanopowder prepared. From the image, the average particle size was estimated to be about 15 nm, which is in good agreement with the result obtained
from XRD analysis.
The nanopowders were uniaxially pressed at 50,000 psi into green pellets of 12.5 mm in diameter and 1 mm in thickness. The pressing process consisted of initially lubricating the die with a die lube, followed by the weighing of an appropriate
amount of powder, inserting the powder in the die, uniaxially pressing to the desired pressure, holding at that pressure for 30 seconds, and then slowly releasing the pressure over 15 seconds. Subsequently, the pellet was forced out from the die. No
binder was added for the forming process. It was found that from the nanopowder the electrolytes can be sintered with greater than 96% of theoretical density at temperatures ranging from 850 to 950.degree. C. In contrast, YSB electrolytes made from
micron-sized powder are typically sintered at temperatures greater than 1,000.degree. C. It is known that the primary driving force for densification of ceramics is the reduction of free surface area at high temperatures. The very small size of the YSB
nanopowder, therefore, has a very large driving force for densification; thus, the required sintering temperature can be significantly reduced relative to commonly used micron-sized powders. This is an important manufacturing advantage of this
invention.
The concept of the invention is also applicable to improve the performance of electrodes for ion conducting materials. These electrodes should have high electrical conductivities, high catalytic activities, adequate porosity for gas transport,
good compatibility with the electrolyte, and long-term stability. In order to achieve high catalytic activities, it is preferred that the electrode be highly porous, so that it retains a large number of active sites for electrochemical reactions, i.e.,
the triple points. Ag has been studied as an electrode material because it is known to have high electrical conductivity and high catalytic activity for oxygen reduction and evolution. However, pure Ag electrodes readily densify during processing and
operation, resulting in a dense electrode with little porosity. In order to reduce the electrode resistance, the teachings of this invention were used to prepare nanocomposite electrodes from Ag and nanostructured powders of the ion electrolyte
material.
EXAMPLE 3
YSB electrolyte pellets, 19 mm in diameter and 0.9 mm in thickness, were sintered from green pellets of 25 mm in diameter and 1.2 mm in thickness. The pellets were ground and polished to a thickness of 0.7 mm to provide a suitable electrolyte
substrate. A separate composite paste of 79 vol % Ag and 21 vol % YSB was prepared by mixing nanopowder of YSB and unfritted Ag paste (marketed by the Cermalloy Division of Heraeus Incorporated of West Conshohocken, Pa., under Catalog No. C440OUF). The
paste was printed onto both sides of the pellet to form electrodes, and then the pellet was fired at 800.degree. C. for 10 minutes to sinter the electrolyte without densifying it beyond the point necessary to provide a robust structure and form a
stabilized sensor cell with porous composite electrodes (about 18% porosity). Then Ag wire was attached to both electrodes with a contact of Ag epoxy which was fired at 730.degree. C. for 2 minutes. It is noted that the composite-electrode/electrolyte
structure needs to be stabilized by the application of heat, pressure or chemical action, as the particular composition of the composite constituents may require or allow, in order to provide a physically robust and stable product.
In the resulting composite electrode structure, we found that the densification of the Ag phase is inhibited by the ion electrolyte material phase. The electrodes then retain a porous microstructure during and after thermally demanding
processing and operation, as shown in the SEM image of FIG. 10. The retained porous microstructure significantly enhances the performance of the electrodes. In addition, electrodes of this kind have better adhesion to the electrolytes because the
stress arising from thermal expansion mismatch between the ceramic electrolyte and the metal electrode is minimized not only by the porous, heterogeneous microstructure of the electrodes but also by tailoring the thermal expansion coefficient of the
nanocomposites. We found that a nanostructured electrode composite sintered to 50-85 percent of full theoretical density (i.e., producing an electrode composite with 15-50 percent porosity) is optimal to obtain these advantages of performance.
Further, another important advantage is derived from using nanocomposite electrodes. If the phases added are ionic conductors or mixed electronic-ionic conductors, the nanocomposite electrode as a whole turns out to be a mixed conductor, which
allows ambipolar transport within the solid phase. FIG. 11 illustrates Nyquist plots of impedance spectra of cells with the nanocomposite electrodes in comparison with a cell with pure Ag electrodes. As determined from the impedance spectra, the
polarization resistances of the nanocomposite electrodes are significantly smaller than that of the pure Ag electrode. As expected, the resistance of the nanocomposite electrode is a function of the composition, i.e., the volume fraction of each
constituent phase. This allows much room for performance optimization by adjusting the composition of the composites. The nanocomposite electrode shows a nearly 4-fold reduction in electrode resistance as compared to the pure Ag electrode. We found
that good results are obtained with a mix of 0 to about 65 vol % electrolyte (35 to 100 vol % metallic electrode material), the limit being that a continuum metal phase must exist for a viable porous electrode structure. That is, the amount of
electrolyte must not be so great as to cause interruptions in the connectivity of the metal phase. At least 5 vol % electrolyte, 21 vol % being preferred, produced good results with different metal/electrolyte combinations. We believe that with
composition optimization, further reduction in electrode resistance can be achieved, leading to a significant enhancement in ion conducting device's performnance.
A sensor produced with the structure manufactured in Example 3 was operated in the following configuration, using air as the reference gas:
The sensor response to the changes in the gas composition and in temperature was monitored by measuring the cell voltage under different conditions.
Shown in FIG. 13 is the sensor voltage response in oxygen as a function of temperature in comparison with that of a conventional YSZ sensor. FIG. 13 clearly shows that the response of the YSB sensor follows Nernst behavior down to 400.degree.
C., while the response of the YSZ sensor deviates from Nernst behavior below 500.degree. C. This indicates that the YSB proof-of-concept sensor can be operated at a temperature about 100.degree. C. lower than conventional YSZ sensors.
FIG. 12 further shows electrode resistances of nanocomposite electrodes as a function of temperature, as determined from the impedance spectra measured in air. Also shown in the figure are the data for pure Ag electrode for comparison. As
compared with pure Ag electrode, the nanocomposite electrodes show significantly lower resistances. The ionic conductivity of the nanostructured YSB electrolyte was measured and found to be over two orders of magnitude higher than that of YSZ
electrolyte, as shown in FIG. 14.
The impedance measurements and the data shown in these examples establish that nanostructured solid ion electrolytes and electrodes are indeed significantly superior in performance to solid ion electrolytes and electrodes prepared from
micron-sized powders. The invention reduced to practice the use of nanostructured ion-conducting electrolytes exhibiting ion conductivity higher than obtained by prior-art technology; and it demonstrated the successful fabrication of ion-conducting
electrolytes in general, and oxygen-conducting electrolytes in particular, from materials with grain size less than 100 nm for electrochemical, electrosynthesis and electrocatalytic applications. It is noted that the methods of assembly or deposition of
nanoparticles to form structures according to this invention may vary depending on the particular application. For example, dry particles may be pressed into a structure of predetermined geometry, as illustrated in Example 1, or deposited over a
supporting substrate according to known vapor deposition processes, as described in U.S. Pat. No. 5,520,040 and illustrated in FIG. 15.
The process of deposition may also be incorporated with the process of manufacture of the nanosize particles disclosed in the referenced copending applications. This method is preferred because it enables the continuous fabrication of product
from raw material. A thermal reactor system is used to produce nanoscale powders by ultra-rapid thermal quench processing of high-temperature vapors through a boundary-layer converging-diverging nozzle. A gas suspension of the micron-sized material is
continuously fed to a thermal reaction chamber and vaporized under conditions that minimize superheating and favor nucleation of the resulting vapor. The high temperature vapor is quenched by passing the vapor stream through the nozzle immediately after
the initial nucleation stages, thereby rapidly quenching it through expansion at rates of at least 1,000.degree. C. per second, preferably greater than 1,000,000.degree. C. per second, to block the continued growth of the nucleated particles and
produce a nanosize powder suspension of narrow particle-size distribution. A gaseous boundary-layer stream is preferably also injected to form a blanket over the internal surface -of the nozzle to prevent vapor condensation in the throat of the nozzle.
A receiving substrate is placed in the diverging section of the nozzle to receive the nanoparticles produced in the quenched stream. Thus, a nanostructured layer of electrolyte material can be deposited directly as desired on the particular device being
manufactured. As those skilled in the art would readily understand, the precise location of the substrate within the nozzle, the residence time, and other operating parameters could be manipulated to produce the physical structure desired for a
particular application.
Potential applications of the invention include nanostructured solid electrolyte and electrode based devices for energy storage and generation such as, but not limiting to batteries, fuel cells, devices for thermodynamic property measurements;
electrochemical sensors for monoatomic, diatomic and polyatomic gases such as, but not limiting to atomic oxygen found in atmosphere, diataomic oxygen and ozone sensors; ion sensors; oxygen pumps; solid state chemical pumps; monitors for steam
electrolyzers; measurement of dissolved oxygen in liquid metals; measurement of pH; electrocatalysis, electrosynthesis, catalytic membrane reactors, and high-temperature kinetic studies. Therefore, while the present invention has been shown and
described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is therefore not to be limited to the details disclosed herein but is
to be accorded the full scope of the claims so as to embrace any and all equivalent apparatus and methods.
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